Further, this application claims priority rights based on German Patent Application No. 198 59 725.8, filed Dec. 23, 1998, which is incorporated herein by reference in its entirety.
The invention relates to an apparatus and a method for examining objects. In particular, the invention relates to an apparatus and a method for examining deformations occurring in objects with diffusely scattering surfaces.
Nondestructive tests of objects are of practical value particularly wherever quality tests of workpieces or other kinds of work objects are to be performed. One practical example is examining tires for defects.
To that end, German Patent DE 42 31 578 C2 describes illuminating the surface of a test object with laser diodes. Observation of the test object is done using an interferometer, which generates an interference pattern on a pattern sensor system. A mirror of the interferometer is movably disposed. The interference patterns generated are 2xcfx80-modulated; this means that a phase rotation exceeding 2xcfx80 can not readily be distinguished from a corresponding phase rotation that is 2xcfx80 less. As a result, a pattern of dots or lines can be generated that characterizes the deformation of the surface.
A plurality of laser diodes are used to light the surface of the object, and each laser diode produces one light spot. The light spots border on one another but overlap, if at all, only in peripheral zones. The lighted portions of the surface taken together make up the total surface. The projection quality of the system is limited.
With this as the point of departure, it is the object of the invention to create an apparatus and a method for observing the surfaces of objects with improved picture quality.
The apparatus of the invention has a measurement head, which generates an interference pattern from light backscattered from the surface of an object. An electronic pattern sensor, which can be integrated with the measurement head, detects the interference pattern and converts it into corresponding electrical signals that can then be further analyzed.
The image field detected by the measurement head is illuminated by a lighting unit, which has a plurality of laser diodes. The laser diodes are disposed such that they form a common light spot. This spot is preferably uniformly lighted, so that within the light spot only slight differences in brightness may be encountered. This is a result of a relatively major overlap of the beams emerging from the individual laser diodes. The common light spot is not subdivided into individual light spots. A plurality of laser diodes illuminate the entire area of the light spot in such a way that radiation backscattered from each point is incoherent. The laser diodes are arranged such that the backscattered total radiation of each portion of the total area observed when the laser diodes are all in operation is greater than when only one laser diode is in operation.
The overlap zones of the regions illuminated by the laser diodes are preferably larger than the nonoverlapping zones. This makes it possible to achieve a uniform distribution of light. Furthermore, it is preferable not to permit any zone that is lighted by only one laser diode. To ensure that no portion of the area is lighted by only one light source (laser diode), laser diodes can also be combined into groups and aimed in groups of two or more laser diodes at a selected portion of the area.
By means of the uniform illumination of the surface of the object to be examined, enhanced picture quality is achieved despite the lack of coherence of the individual components of the light striking the surface. In the interference pattern generated, the desired deformations are readily apparent both in the middle of the image and at the edges.
The beams of the laser diodes can overlap so markedly that more than half the area of the light spot receives light from two laser diodes. There may exist extended regions that are lighted with approximately the same intensity from the light of a plurality of laser diodes. The light spots of individual laser diodes can, as a result, occupy virtually the entire field to be illuminated. In this way, narrow edge zones in which the light spots adjoin one another, and where uneven light distribution could occur, are avoided.
There can also be a plurality of regions that are struck by the light from more than two laser diodes. The orientation is expediently arranged in most cases to provide a uniform distribution of brightness. It is also possible to have virtually the entire area of the light spot illuminated by more than two diodes or by other numbers of diodes.
From independent light sources, not coherent with one another but oriented virtually in the same direction, a plurality of coincidence speckle fields can be generated and projected simultaneously onto a pattern sensor. The resultant superimposed speckle field is detected and used to calculate the deformation of the object.
It is possible to have the laser diodes of the entire group shine simultaneously, preferably in continuous operation. In a modified embodiment, the laser diodes can be operated in pulsating fashion. This makes a higher light yield possible at the moment a picture is taken; as a result, either the object field under observation can be enlarged, or the lighting power or the exposure time can be reduced.
It is advantageous if the individual light sources are not coherent with one another, but have light wavelengths differing only slightly from one another. Furthermore, the angles of incidence (the angles at which the beams of light strike the object) of the individual light sources should not differ excessively.
It is also possible to use somewhat more-different light wavelengths, say, xcex1 and xcex2. If two groups of light sources are used, the resulting sensitivity can then be calculated as
xcexres=(xcex1*xcex2)/(xcex1+xcex2).
Each wavelength component should be present with, as much as possible, the same intensity.
Alternatively, it is possible to trigger the laser diodes or to provide them with a shutter device, for instance, in such a way that the light beams of the laser diodes strike the surface of the object in a chronologically staggered fashion. The resulting individual interferograms can be superimposed at the pattern sensor and added together (integrated), or depending on the hardware, they can be detected individually and then combined with one another in a computer. As in the case of continuous lighting, the overall result is a uniformly illuminated field. The laser diodes can be disposed such that they are either stationary or in motion.
The pattern sensor is preferably connected to an image analysis device, which on the basis of a plurality of detected interference patterns determines a deformation of the surface of the object. This is expedient especially in cases in which the structure or form of the undeformed surface of the object is of no interest. Such measurements are expedient, for example, in workpiece testing or materials testing. For instance, they can be employed to detect defects in tires. The surface of the tire to be examined is detected at two ambient pressures different from one another. The resultant deformations are rendered visible.
The interferometer can function without a direct interference beam. This is possible if the beam backscattered from the object is split into two fractional beams, one of which is subjected to a phase displacement. The phase displacement can preferably be controlled or monitored.
To that end, it is advantageous to employ a device for phase displacement, preferably, a mirror that is adjusted by a piezoelectric actuator. It is possible to use a Michelson interferometer as the interferometer; however, an especially advantageous embodiment uses an arrangement in which the object beam, received at the measurement head, is split into two fractional beams that reach the pattern sensor over different paths and reunite only there. This has the advantage of avoiding the type of light losses that occur in the Michelson interferometer when the fractional beams are reunited in the beam splitter.
The pattern sensor is connected to an image analysis device that preferably has a computation unit. The computation unit, preferably a suitably powerful computer, executes a program that performs image processing. For instance, from a plurality of pictures taken of the stationary object with mutually shifted phase relationships, a phase image is calculated. The phase relationships of individual pixels are, as a rule, stochastically distributed and do not provide any direct conclusion that can be drawn about the object. If the surface of the object is deformed, however, or if it is displaced by a slight distance toward or away from the measurement head, and if in this state a phase image is obtained, for instance, by linking together a plurality of interference patterns altered by phase displacement, then a phase difference image can be derived from the two phase images. The phase difference image provides information on local displacements or deformations and can be displayed. To that end, the applicable phase difference angle of each pixel is assigned a gray value that is displayed at the appropriate point on a playback device. For example, the display value of black may be assigned to a phase difference angle of 0, and the gray value of white may be assigned to the phase difference angle of 2xcfx80.
In an advantageous feature of the method and the associated apparatus, the phase difference angles, before they are displayed, are 2xcfx80/n-modulated. To that end, the differential angles of the phase difference image are subjected, on a pixel-by-pixel basis, to a modulo 2xcfx80/n division. This means that the phase difference angle is divided by 2xcfx80/n, and the remainder is the result. This result is multiplied by the factor n and yields the 2xcfx80/n-modulated value, which in the range from 0 to 2xcfx80/n is displayed with gray values ranging between black and white. If desired, a color display may also be selected. The factor n is preferably an integer greater than 1. This amplifies any deformations of the object that are present and visible in the phase difference image and thus makes defects in the object more clearly apparent.
Alternatively, it is possible to use the phase difference or the 2xcfx80/n-modulated values of the phase differences to modulate a sine function and to have the resulting value displayed. This produces a stripe pattern that characterizes the object deformation. The higher the value of n selected, the higher the resolution; that is, the more stripes appear in a particular object deformation. It is expedient if the user can select the factor n by suitable input means. The factor can also be switched over, for instance after one picture is taken, so that the same test run, or, in other words, the same object deformation, can be represented using different modulations.
The method is suitable for testing objects with diffusely scattering surfaces and produces a picture-type illustration of phase difference angles. This allows the user to easily recognize structural defects in a measured object. This method can be employed for arbitrary test objects or sizes of defects and requires a relatively low computational effort, so that display of the results virtually in real time is assured.
In another advantageous feature of the invention, the value to be displayed is scaled in such a way that the full gray value range that the image processing system is capable of displaying is utilized. To make defects more visible, the gray value or color value corresponding to an angle of magnitude zero can be interactively set by the user.
The 2xcfx80/n-modulated phase difference image can also be obtained directly from the phase-displaced intensity patterns of a first imaging series for a first object state and a second phase-displaced imaging series for a second object state. To this end, the equations for the 2xcfx80/n-modulation are inserted into the equations for generating the phase difference images from the intensity patterns.
In an advantageous embodiment, all the points in the light spot that are covered by the camera or other imaging device are lighted by at least two laser diodes. As a result, correct measurement results can be obtained even if one laser diode has failed. The light is then incoherent in the entire range covered by the camera.
Further details of advantageous features of the invention are the subject of the drawings, the description and the claims.